1. Field of the Invention
The present invention relates to a method for producing an image. More particularly, it relates to a method for producing a heat resistant, high contrast image having high resolving power using a photographic material which cmprises a substrate having thereon at least one silver halide emulsion layer, either directly or on at least one subbing layer on the substrate. It also relates to a method for easily producing a durable photomask having high resolving power and good edge sharpness using a photographic material which comprises a transparent substrate having a masking layer thereon, the masking layer having thereon at least one silver halide emulsion layer, either directly or on at least one subbing layer on the masking layer.
2. Description of the Prior Art
The optical density of a silver image formed on a photographic material by exposing and developing a photographic material which comprises a silver halide emulsion layer coated on a substrate gradually decreases from its maximum value to a background value at the edge of the silver image. The spacing between the maximum image density portion and the background is usually about 1 micron. Therefore, it is difficult to obtain a high contrast silver image having closely separated about 1 micron lines or spacings. Silver grains existing between adjacent image lines reduce the image contrast and resolving power.
Moreover, since such an emulsion layer is colored due to thermal decomposition of the binder when heated to about 150.degree. to 200.degree. C, it can not be used for purposes requiring heat resistance.
One field which requires a heat resistant image is "super-microphotography". An image reduced on a 35 mm film from a 9 by 14 inch (23 by 36 cm) size original with a reduction ratio of about 10 is usually called a "microphotograph", and an image further reduced (about 2 by 3 mm) by a factor of about 10 is called a "super-microphotograph". A microphotograph can thus be considered to be an image reduced by a factor of about 10 and a super-microphotograph an image reduced by a factor of about 100.
Since the image size of a super-microphotograph is about 2 by 3 mm or smaller, the enlarging factor is about 100 (1000 based on area ratio) when a super-microphotograph is projected on a screen to provide the original image size. Consequently, a light intensity of about 10 million lux is necessary on the image surface of the super-microphotograph if the image projected on a transmission type screen, e.g., with a blackened back surface, is to have a light intensity of 100 lux when the screen has a transmission optical density of 1. In fact, the super-microphotographic image is illuminated with a light intensity of about 12 to 13 million lux to compensate for the loss of the projection lens. The temperature of the emulsion layer of the super-microphotographic increases to several hundred degrees C, due to the heat generated by the light absorbed in the emulsion layer, when it is continuously illuminated with such a strong light. As a result, the binder of the emulsion layer is thermally decomposed and colored to cause the image projected on the screen to be dim and colored. Since the silver image areas absorb light well, the temperature of these areas preferentially increases and the binder of these areas is preferentially decomposed, whereafter the decomposition spreads into the surrounding areas. Decomposition of the binder in even the non-silver image areas proceeds in an accelerated manner once it is slightly colored and light absorption occurs.
Heretofore, emulsion masks and hard masks have generally been used as photomasks in microelectronic manufacturing processes. However, an emulsion mask has low edge contrast, as described above, and such low mechanical strength that it is easily damaged, that is, durability is poor. On the other hand, a hard mask is quite durable, but the process for production thereof is complicated. Also, the production of a hard mask requires a photoetching process that uses a photoresist which has low sensitivity and requires long exposure times.